1. Field of the Invention
The present invention relates to a system and method for fabricating a pre-stressed modular construction for supporting or retaining an applied load. More particularly, the present invention relates to a system and method for pre-stressed modular retaining walls.
2. Related Art
A retaining wall is an engineered structure that has the particular task of ensuring that a given unstable, or potentially unstable, soil mass is prevented from moving under the influence of gravity. Frequently, the retaining wall is also called upon to withstand a superimposed load, a surcharge load, on and/or within the soil mass, such as a highway, together with its traffic loading, or the loading induced by the foundations of a building located in close proximity to the retaining structure. Further, the retaining wall may be required to support some other non-retaining load that is resisted by structural elements directly attached to, and/or incorporated within, the wall structure itself
Since the early 1970's, numerous alternative wall systems have been introduced. Examples of these systems include mechanically stabilized earth (MSE) walls and reinforced soil slopes (RSS) employing metallic or polymeric internal reinforcement; anchored walls, such as the soldier pile and lagging walls, diaphragm walls, and soil mixed walls; prefabricated modular gravity wall systems including cribs, bins, and gabions; and in-situ reinforced wall systems such as soil-nailed walls and micropile walls. However, because of the ever increasing demands that are being placed on our city and urban environments and, most noticeably, on the country's transportation infrastructure, together with the need to preserve our natural environment while providing for the associated societal expectations, there is an increasing number of problematic sites where the currently available retaining wall options cannot provide an optimal solution. In particular, for those sites that require “foundation-up” construction, there is a dearth of rapid construct, high capacity retaining wall systems possessing significant functional flexibility and which demand only a small construction footprint. Retaining structures constructed to resist soil pressures are often categorized according to their basic mechanisms of retention. The retention mechanisms include internally stabilized, externally stabilized, and hybrid systems. Alternatively, retaining walls may be categorized according to their source of support, that is, their source of equilibrating reaction forces, The sources of support for these retaining walls may be bracketed into gravity, semigravity, and nongravity.
An internally stabilized system involves reinforced soils to retain a soil mass and any surcharge loads. This reinforcing may be provided by adding reinforcement directly to the soil mass, where this augmented soil mass is providing the retaining/self-retaining structure, as the system is being constructed from the “ground” up. Various types of reinforcement are available, and the soils between the layers of reinforcement are placed in a carefully controlled manner meeting design specifications—that is, the placed soil is “engineered fill.” Frequently, pre-cast concrete elements are tied directly to these soil reinforcing components. This system forms the basic approach of Mechanically Stabilized Earth, MSE, retaining wall systems.
Alternatively, this internal stabilization via the reinforcing of the soil mass in question may proceed from the top down. In this (directionally) opposite approach, reinforcing elements are added to the existing soil mass in order to provide the existing materials with a greater degree of internal stability. As an example of this approach, the face, that is exposed as the excavation proceeds from the top down, has soil nails installed through it into the ground mass, which nails extend beyond any potential failure plane. Often, a shotcrete cover over the exposed face is placed and subsequently connected to these nails, thereby providing a protection against erosion of the soil face.
Further to the above methods of reinforcing a soil mass, driven piles or cast-in-drilled-hole piles may be used to stabilize the mass of concern. However, this approach is generally considered when the stability issue is more global in nature. By “global” is meant the situation where a body of soil is experiencing a deep-seated instability, which instability ideally needs to be eliminated.
With externally stabilized systems, a physical structure is employed to confine the body of soil. The equilibrating reaction forces, required by an externally stabilized system, are provided either through the weight of a morpho-stable structure, or by the reactions mobilized via the inclusion and/or extension of various system elements into “reaction zones”. The latter reactions may be generated by driving the piles of a sheet-pile wall system, for example, to sufficient depths into competent soil. Or, reactions may be generated via the use of ground anchors providing point-reactions on the externally stabilizing structure. Frequently, combinations of reaction-force-providing structural elements are employed, in a given situation, to deliver the total force equilibration required for an externally stabilized retaining wall.
With regard to sources of support, that is, with regard to the sources of the equilibrating reaction forces, retaining wall systems may be categorized into three groups. These are the groupings of (1) gravity walls, (2) semigravity walls, and (3) nongravity walls.
Gravity walls derive their capacity to resist imposed soil loads through the dead weight of the wall itself (that is the physical wall that is constructed) or through an integrated mass that can be either internally or externally stabilized. Gravity walls may be further classified into four types as follows. The first type is an internally stabilized soil mass system. Some of the examples given above are typical. The stability of a cut slope may be maintained in a top-to-bottom installation of soil nails, installed as the excavation of materials proceeds. Or, a retaining soil mass may be constructed of engineered fill, in a bottom-to-top sequence, thereby creating a soil mass possessing the required internal stability via the inclusion of reinforcing elements at regular vertical spacing. Where the soil mass is constructed from engineered fill, the face of such soil mass may be protected by using pre-cast concrete facings as with many MSE systems. Where soil nails are used, the front face is preferably protected using shotcrete or cast-in-place concrete. The second type of gravity wall is an externally stabilized soil mass system. Included in this category are simple modular pre-cast concrete walls. Such simple pre-cast concrete walls are stacked, but include no internal mechanism for enhancing structural capacity. Another example is prefabricated metal bin walls. The third type is also an externally stabilizing system. In this category are the generic walls including the masonry walls, the stone walls, “dumped” (usually shaped) rock walls, and the contained rock walls, often using uniform crushed rock and known as gabion walls. The fourth system is also an externally stabilizing system. Examples are the use of cast-in-place mass concrete wall, or the cement-treated soil wall. Where the face of the treated soil wall requires protection, a pre-cast concrete panel may be used, which panel would be anchored to the treated-soil wall.
Semigravity walls derive their restraining capability through the combination of dead weight and structural resistance. Generally, these semigravity walls are externally stabilizing structures. They may be constructed on spread footings or on deep foundations. Historically, the dominant type of semigravity retaining wall is the conventional cast-in-place concrete cantilever structure. Alternatively, various kinds of pre-cast concrete walls are available in the market, which walls are constructed on cast-in-place footings. Cantilever semi-gravity retaining walls may be very reliant on the dead weight of the soil mass that rests on the section of the foundation footing that extends back beyond the wall's stem, while also developing the necessary structural resistance. An example of the necessary structural resistance would be the wall's moment and shear capacity at the base of the stem.
Nongravity walls derive their restraining capability through lateral resistance. This lateral resistance may be mobilized in a number of ways. For example, the continuation of vertical structural elements down to competent soils, or the use of ground anchor retainers directly delivering point resistance to the retaining structure. Examples of externally stabilizing nongravity systems are embedded cantilevering wall elements, sheet piles, drilled shafts, or slurry walls. A second group of nongravity walls includes the first listing of embedded walls but have additional restraint via utilizing multiple ground anchor retainers.
Where, for example, there is a need to arrest the creep movement of a slope, nongravity systems may be employed in the form of dowel piles or caissons, to internally stabilize the soil mass. It should be noted that required equilibrating forces may be developed via the use of reaction members which develop point-reaction-forces. (Consider the reactions to a truss, which truss transfers moment to its support). That is, the structural elements delivering resistance to the retaining wall structure overall may have so little moment (and shear) resisting capacity, if any, that the equilibrating set of forces are established via point-acting reaction forces. For example, an arrangement of elements for such a system, may consist of a set of vertical (or near vertical) piles, a set of (near) vertical ground anchors and, finally, a set of (near) horizontal ground anchors. In this case, the piles would take up compression loads, the (near) vertical ground anchors would provide a (predominantly) downward reaction, which would act in concert with the piles' upward reaction to provide moment resistance to the base foundation. The (near) horizontal ground anchors, placed appropriately at the foundation beam/pile cap level, would resist the net “shear” forces from the retaining wall structure that would cause the foundation element to translate.
An example of a retaining wall is shown, for example, in U.S. Pat. No. 2,149,957 (“the Dawson patent”). The wall of the Dawson patent utilizes stretchers and headers to construct a retaining wall. Dawson further discloses “positive tensile anchorage.” Such “positive tensile anchorage” refers to the construction of the individual elements and has no impact on the primary behavior of the system disclosed in the Dawson patent. Moreover, the wall of the Dawson patent does not pre-stress header assemblies through post-tensioning. Further, the Dawson patent does not disclose vertically disposed passive reinforcement through the header assemblies.
Retaining wall systems, such as those shown in the Dawson patent, often do not provide an optimal solution for retaining or supporting an applied load. The design of conventional retaining wall systems may result in constructibility problems, resulting in longer construction periods, higher cost, and more extensive use of the surrounding land. Thus there is a need in the art for a retaining wall system that provides an improved solution for retaining or supporting an applied load and overcomes the limitations of constructibility problems with existing systems. There is a further need in the art for a retaining wall system that is modular and adaptable to a wide variety of construction needs.